White blood cells called T lymphocytes play critical roles in immune defense against viruses, bacteria, fungi, protozoa, and cancer cells. They are also involved in allergies/asthma due to the development of an unwanted or excessive type of immune response to substances in our environment and in autoimmune diseases that result from the inappropriate attack of these cells on the body?s own tissues. The effector functions of T cells are mediated largely by proteins termed cytokines that either be expressed at the cell surface or secreted. Because T cells see foreign substances (antigens) in the form of peptide-major histocompatibility complex (MHC) molecule complexes on cell surfaces, we wish to know how such complexes interact with specific receptors to evoke the effector activities of mature T cells in the body, as well as regulate their growth, inactivation, or death. In particular, we want to understand in molecular detail the protein- protein interactions that turn recognition of antigen by T cells into signals guiding the normal survival and effector functions of these cells, how variations in these recognition and signaling events leads to desirable versus undesirable forms of immunity, and how we can manipulate these events to augment useful immune responses and inhibit damaging ones. Our studies currently focus on the intracellular events evoked by binding of the T-cell receptor with peptide:MHC molecule ligands and on new biochemical regulatory pathways that help T-cells discriminate between self- and foreign peptide:MHC molecule complexes, as well as show very high sensitivity to antigen on presenting cells of the types studied in LI545. During the past year we have discovered the existence of two novel regulatory pathways controlling early signaling by the T-cell receptor (SHP-1 phosphatase dependent negative control and MAPK mediated positive control) . We have also demonstrated a role for self-recognition in controlling the viability of mature na?ve peripheral T-cells. - T-cell receptor; MHC molecule; signal transduction; lymphocyte homeostasis.